In 4D ultrafast electron microscopy (UEM), ultrafast light pulses generate electron packets by photoemission at the cathode of an electron microscope. The electron packets are then used to probe dynamic processes initiated by heating or by exciting the microscopic specimen with a second, synchronized ultrafast light pulse. In conventional implementations, each pump pulse on the specimen is accompanied by one suitably delayed laser pulse on the cathode to generate one packet of electrons probing a single time point in the evolution of the specimen. A record of the full course of the temporal evolution of the specimen is then constructed by repeating the experiment multiple times with variation of the delay time between the two light pulses, reading out a separate CCD image for each delay time. Thus, information about different time points in the dynamic response of the specimen is obtained from different excitation events.
Conventional techniques are well suited for specimens that undergo irreversible, but sufficiently well-defined dynamics, to allow a new specimen area to be used for each time point or for a specimen that recovers fully to allow repeated identical excitations of the same area.
Despite the progress made in the field of UEM, there is a need in the art for improved methods and systems related to electron microscopy.